Class template cpp_dec_float
fulfils all of the requirements for a Backend
type. Its members and non-member functions are deliberately not documented:
these are considered implementation details that are subject to change.

The class takes three template parameters:

Digits10

The number of decimal digits precision the type should support. Note
that this type does not normally perform any dynamic memory allocation,
and as a result the Digits10
template argument should not be set too high or the class's size will
grow unreasonably large.

ExponentType

A signed integer type that represents the exponent of the number

Allocator

The allocator used: defaults to type void,
meaning all storage is within the class, and no dynamic allocation
is performed, but can be set to a standard library allocator if dynamic
allocation makes more sense.

The type of number_category<cpp_dec_float<Args...>>::type is mpl::int_<number_kind_floating_point>.